The present invention relates to methods and systems for electro-optically detecting fabrication defects, which are random in nature, in semiconductor patterned structures such as a semiconductor wafer featuring integrated circuit dies or chips. In particular, the present invention relates to a method and system for fast on-line electro-optical detection of wafer defects by illuminating with a short light pulse from a pulsed laser, a field of view of an electro-optical camera system having microscopy optics, and imaging, a moving wafer, on to a focal plane assembly (FPA) optically forming a surface of photo-detectors at the focal plane of an optical imaging system, formed from several detector ensembles, each detector ensemble including an array of several two-dimensional matrix photo-detectors, where each two-dimensional matrix photo-detector produces an electronic image featuring a matrix of picture elements (pixels), such that the simultaneously created images from the different matrix photo-detectors are processed in parallel using conventional image processing techniques, for comparing the imaged field of view with another field of view serving as a reference, in order to find differences in corresponding pixels, indicative of the presence of a wafer die defect.
Hereinafter, the term xe2x80x98waferxe2x80x99 refers to, and is generally considered to feature individual patterned structures, known as xe2x80x98semiconductor wafer dicexe2x80x99, xe2x80x98wafer dicexe2x80x99, or wafer chipsxe2x80x99. Current semiconductor technology involves the physical division of a single wafer into identical dies for the manufacture of integrated circuit chips, such that each die becomes an individual integrated circuit chip having a specific pattern, such as a memory chip or a microprocessor chip, for example. The type of chip produced from a given die is not relevant to the method or system of the present invention.
Hereinafter, the term xe2x80x98field of viewxe2x80x99 refers to that part or segment of, a wafer, in general, and a wafer die, in particular, illuminated by a pulsed laser and imaged by the electro-optical camera system inspection optics in conjunction with the FPA. Accordingly, an entire single wafer die, and therefore, an entire single wafer featuring a plurality of wafer dies, is inspected by sequential imaging of a plurality or sequence of fields of view. The field of view can be considered as the inspection system electro-optical imaging footprint on the wafer or wafer die. Successive fields of view created while the wafer is moving in one direction are referred to as a xe2x80x98stripxe2x80x99 of fields of view. Pixels are referred to with respect to forming an image of a field of view by the electro-optical inspection system. As a reference dimension, general order of magnitude of the size of a typically square wafer die within a wafer is 1 centimeter by 1 centimeter, or 104 microns by 104 microns.
Hereinafter, detection of a xe2x80x98wafer defectxe2x80x99 refers to the detection of the presence of an irregularity or difference in the comparison of like patterns of wafer dies or like patterns of fields of view. Current methods and systems of defect detection on wafers are usually based on the analysis of comparing signals obtained from a number of adjacent wafer dies or fields of view, featuring a like pattern. Defects produced during wafer fabrication are assumed to be random in nature. Therefore, defect detection is based on a statistical approach, whereby the probability that a random defect will exist at the same location within adjacent wafer dies is very low. Hence, defect detection is commonly based on identifying irregularities through the use of the well known method of die-to-die comparison. A given inspection system is programmed to inspect the pattern of a wafer die or field of view, typically referred to as the inspected pattern, and then compares it to the identical pattern of a second wafer die or field of view on the same wafer, serving as the reference pattern, to detect any pattern irregularity or difference which would indicate the possible presence of a wafer defect. A second comparison between the previously designated inspected pattern and the like pattern of a third wafer die or field of view is performed, in order to confirm the presence of a defect and to identify the wafer die or field of view containing the defect. In the second comparison, the first wafer die or field of view is considered a reference and the third wafer die or field of view is considered as inspected.
Fabrication of semiconductor wafers is highly complex and very expensive, and the miniature integrated circuit patterns of semiconductor wafers are highly sensitive to process induced defects, foreign material particulates, and equipment malfunctions. Costs related to the presence of wafer defects are multiplied several fold when going from development stages to mass production stages. Therefore, the semiconductor industry critically depends on a very fast ramp-up of wafer yield at the initial phase of production, and then achieving and controlling a continuous high yield during volume production.
Critical dimensions of integrated circuits on wafers are continuously decreasing, approaching 0.1 micron. Therefore, advanced semiconductor wafers are vulnerable to smaller sized defects than are currently detected. Current methods of monitoring wafer yield involve optically inspecting, in-process, wafers for defects and establishing a feedback loop, with appropriate parametric process control, between the fabrication process and the manufactured wafers. To detect smaller sized defects, optical inspection systems need to have higher resolution via scanning wafers using smaller pixel sizes. Scanning a given sized wafer using pixel sizes smaller causes an increase in per wafer inspection time, resulting in decreased wafer throughput, and decreased statistical sample sizes of the number of inspected wafers. Conversely, attempting to increase wafer inspection throughput by using current optical system pixel sizes results in reducing the effectiveness, i.e., resolution, of detecting wafer defects.
In addition to decreasing critical dimensions of wafers, the semiconductor industry is in the process of converting from manufacturing 8-inch wafers to 12-inch wafers. Larger, 12-inch wafers have more than twice the surface area compared to 8-inch wafers, and therefore, for a given inspection system, inspection time per 12-inch wafer is expected to be twice as long as that per 8-inch wafer. Fabricating 12-inch wafers is significantly more expensive than fabricating 8-inch wafers. In particular, costs of raw materials of 12-inch wafers are higher than those of 8-inch wafers. One result of wafer size conversion, is that cost effective productivity of future wafer manufacturing will depend critically upon increasing speed and throughput of wafer inspection systems.
Automated wafer inspection systems are used for quality control and quality assurance of wafer fabrication processes, equipment, and products. Such systems are used for monitoring purposes and are not directly involved in the fabrication process. As for any principle component of an overall manufacturing system, it is important that a wafer inspection method, and system of implementation, be cost effective relative to the overall costs of manufacturing semiconductor wafers.
There is thus a need to inspect semiconductor wafers for wafer die defects, for wafers featuring larger sizes and smaller critical dimensions, at higher throughput than is currently available, and in a cost effective manner.
Automated optical wafer inspection systems were introduced in the 1980""s when advances in electro-optics, computer platforms with associated software and image processing made possible the changeover from manual to automated wafer inspection. However, inspection speed, and consequently, wafer throughput of these systems became technology limited and didn""t keep up with increasingly stringent production requirements, i.e., fabricating integrated circuit chips from wafers of increasing size and decreasing critical dimensions. Current wafer inspection systems typically use continuous illumination, and create a two dimensional image of a wafer segment, by scanning the wafer in two dimensions. This is a relatively slow process, and as a result, quantity of on-line inspection data acquired during a manufacturing process is small, generating a relatively small statistical sample of inspected wafers, translating to relatively long times required to detect wafer fabrication problems. Slow systems of on-line defect detection result in considerable wafer scrap, low wafer production yields, and overall long turn-around-times for pin-pointing fabrication processing steps and/or equipment causing wafer defects.
A notable limitation of current methods and systems of wafer defect detection relates to registration of pixel positions in wafer images. Before wafer defects can be detected by standard techniques of comparing differences in pixel intensities of an image of a targeted or inspected wafer die to pixel intensities of an image of a reference wafer die, the pixel positions of the images of the inspected and reference wafer die need to be registered. Due to typical mechanical inaccuracies during movement of a wafer held on a translation stage, velocity of a wafer beneath a wafer inspection camera system is not constant. As a result of this, image pixel positions in the fields of a detector are distorted and may not be as initially programmed. Therefore, a best fit two-dimensional translation pixel registration correction is performed.
Prior art methods and systems of wafer defect detection, featuring a combination of continuous wafer illumination and acquiring a two dimensional image by either scanning a wafer in two dimensions using a laser flying spot scanner as taught in U.S. Pat. No. 5,699,447, issued to Alumot et al., or scanning a wafer in one dimension using a linear array of photo detectors as taught in U.S. Pat. No. 4,247,203, issued to Levy et al., require a registration correction for all pixels or all pixel lines. These methods limit system speed, i.e. inspection throughput, and require substantial electronic hardware. Moreover, they result in residual misregistration, since no correction procedure is accurate for all pixels in an image. Residual misregistration significantly reduces system defect detection sensitivity. For a wafer inspection method or system in which all focal plane assembly pixels in any given field can be considered one unit, generated simultaneously, there is no need for image pixel registration within a field of view of a focal plane assembly. Therefore, only a single two dimensional alignment correction between the inspected field of view and the equivalent zone in a reference field of view is needed and a single alignment correction will be correct over the entire focal plane assembly field of view. Such a procedure results in negligible residual misregistration, enabling improved defect detection sensitivity. There is thus a need for a method or system of on-line electro-optical detection of wafer die defects which includes minimization of residual misregistration of image pixel positions.
An apparatus for wafer inspection is disclosed in U.S. Pat. Nos. 4,247,203, and 4,347,001, both issued to Levy at al. The apparatus described in those patents locates defects or faults in photomasks by simultaneously comparing patterns of adjacent dies on the photomask and locating differences. Using two different imaging channels, equivalent fields of view of each die are simultaneously imaged, and the images are electronically digitized by two linear diode array photo-detectors, each containing 512 pixels. A two dimensional image of a selected field of view of each die is generated by mechanically moving the object under inspection in one direction, and electronically scanning the array elements in the orthogonal direction. During the detector exposure time, the photomask can not be moved a distance of more than one pixel or the image becomes smeared. Therefore, the time to scan and inspect the photomask is very long. Since the photomask is moved continuously while the two dimensional images are generated, it is necessary that the photomask move without jitter and accelerations. This motion restriction requires a very massive and accurate air-bearing stage for holding and moving the photomask, which is costly. In addition, the wafer inspection apparatus of Levy et al. is capable of detecting 2.5 micron defects with 95% probability of detection on photomasks. For critical dimensions of current semiconductor, integrated circuits approaching 0.1 micron, this means that the inspecting pixel must be of similar size magnitude. Since inspection speed increases inversely with squared pixel size, the apparatus of Levy et al. would slow down by more than two orders of magnitude. Furthermore, it becomes impractical to implement a motion stage capable of meeting the required mechanical accuracies.
Wafer inspection has also been implemented using a single imaging and detection channel, based on a solid state camera using a two dimensional CCD matrix photo-detector, such as described in xe2x80x98Machine Vision and Applicationsxe2x80x99, (1998) 1: 205-221, by IBM scientists Byron E. Dom et al. A wafer inspection system designated as P300 is described for inspecting patterned wafers having a repetitive pattern of cells within each die, such as in semiconductor wafers for memory devices. The system captures an image field of view having 480 by 512 pixels. The image processing algorithms assume a known horizontal cell periodicity, R, in the image, and analyzes each pixel in the image by comparing it with two pixels, one pattern repetition period, R, away in either horizontal direction. Such a comparison of like cells within a single image is called a cell-to-cell comparison. The pixel under test is compared with periodic neighbors on both sides to resolve the ambiguity that would exist if it were compared with only a single pixel. While this system is capable of simultaneously capturing a two dimensional image of the object under test, it is very slow in inspecting an entire wafer. Millions of image fields are needed to image an entire wafer, and since the system uses continuous illumination, such as is used with standard microscopes, the wafer must be moved, under the inspection camera, from field to field and stopped during the image exposure to avoid image smear. To reach another field, the mechanical motion stage carrying the wafer must accelerate, and than decelerate to a stop at a new position. Each such motion takes a relatively long time and therefore inspecting a wafer typically takes many hours.
There is thus a need for, and it would be useful to have, a fast on-line method and a system to inspect semiconductor wafers for wafer die defects, for wafers featuring larger sizes and smaller critical dimensions, at higher throughput than is currently available, while providing high levels of image resolution of water dies, in a cost effective manner.
The present invention relates to a method and system for fast on-line electro-optical detection of wafer die defects by illuminating with a short light pulse from a pulsed laser, a field of view of an electro-optical camera system having microscopy optics, and imaging a moving wafer, on to a focal plane assembly optically forming a surface of photo-detectors at the focal plane of an optical imaging system, formed from several, for example six detector ensembles, each detector ensemble including an array of several, for example, four, two-dimensional charge coupled device (CCD) matrix photo-detectors, whereby each two-dimensional CCD matrix photo-detector produces an electronic image containing a large matrix of, for example, two million, pixels, such that the simultaneously created images from the different CCD matrix detectors are processed in parallel using conventional image processing techniques, for comparing the imaged field of view with another field of view serving as a reference, in order to find differences in corresponding pixels, indicative of the presence of a wafer die defect.
In particular, the method and system of the present invention enable capturing high pixel density, large field of view images of a wafer die, on-the-fly, without stopping movement of the wafer. High accuracy of wafer motion speed is not needed, and a relatively simple inexpensive mechanical stage for moving the wafer can be used. The continuously moving wafer is illuminated with a laser pulse of such short duration, for example, ten nanoseconds, significantly shorter than the image pixel dwell time, that there is effectively no image smear during the wafer motion. During the time interval of the laser pulse, a wafer die image moves less than a tenth of a pixel. The laser pulse has sufficient energy and brightness to impart the necessary illumination to the inspected field of view required for creating an image of the inspected wafer die. In addition, as a result of the method and system featuring optical coupling of the separate CCD matrix photo-detectors via the detector ensembles and the focal plane assembly, processing time of an entire array of for example twenty-four CCD matrix photo-detectors, having imaging capacity of 48 megapixels, is equivalent to processing time of a single CCD matrix photo-detector of the order of {fraction (1/30)} of a second, since the processing of all the photo-detectors is processed in parallel. Consequently, parallel processing of the entire focal plane assembly including twenty-four CCD matrix photo-detectors provides an overall pixel processing data rate of nearly 1.5 gigapixels per second. Furthermore, the overall wafer inspection system operates essentially at 100% efficiency) whereby, the laser pulse rate of 30 pulses per second is synchronized with the frame speed of 30 frames per second of each CCD matrix photo-detector, and the wafer is moved at a linear speed such that the distance between successive fields of view is covered in {fraction (1/30)} of a second.
The method and system of the present invention provide significant improvements over currently used methods and systems for electro-optical inspection and detection of wafer defects, in the semiconductor wafer fabrication industry, including providing high resolution large field of view wafer die images at very high wafer inspection throughput, and requiring less electronic and system hardware. Moreover, as a direct result of using an array of several CCD matrix photo-detectors for acquiring a high pixel density image of a wafer die illuminated by a single light pulse, the method and system of the present invention prevents misregistration of pixel positions in the wafer die images, enabling enhanced defect detection sensitivity. Such a method and system of wafer defect detection results in faster, more efficient, and cost effective, feedback control of wafer fabrication processes.
Thus, according to the present invention, there is provided a method for electro-optically inspecting a patterned semiconductor wafer of dies for a defect, the method comprising the steps of: (a) moving the patterned wafer along an inspection path; (b) providing a repetitively pulsed laser illuminating source; (c) sequentially illuminating each of a plurality of fields of view in each of a plurality of the wafer dies by using the pulsed laser illuminating source; (d) sequentially acquiring an image of the each of the plurality of the sequentially illuminated fields of view in each of a plurality of the wafer dies by using an electro-optical camera including at least two two-dimensional matrix photo-detectors, the at least two two-dimensional matrix photo-detectors simultaneously acquiring images of each of the plurality of the sequentially illuminated fields of view in each of a plurality of the wafer dies; and (e) detecting a wafer defect by comparing the sequentially acquired images of each of the plurality of the sequentially illuminated fields of view in each of a plurality of the wafer dies using a die-to-die comparison method.
According to still further features in the described preferred embodiments, the repetitively pulsed laser is a Q switched Nd:YAG laser.
According to still further features in the described preferred embodiments, the Q switched Nd:YAG laser is optically pumped by light emitting diodes.
According to still further features in the described preferred embodiments, the electro-optical camera further includes a non-linear optical crystal functioning as a second harmonic generating crystal, placed in a laser beam light path of the repetitively pulsed laser illumination source, the non-linear optical crystal halving wavelengths of the laser beam light generated by the repetitively pulsed laser.
According to the present invention, there is provided a system for electro-optically inspecting a patterned semiconductor wafer of dies for a defect, the system comprising: (a) a mechanism for providing movement of the patterned wafer along an inspection path; (b) a repetitively pulsed laser illumination source for illuminating the patterned wafer; (c) an electro-optical camera including at least two two-dimensional matrix photo-detectors for sequentially acquiring an image of each of a plurality of sequentially illuminated fields of view in each of a plurality of the wafer dies, the at least two two-dimensional matrix photo-detectors operate with a mechanism for simultaneous acquisition of images of each of the plurality of the sequentially illuminated fields of view in each of a plurality of the wafer dies; and (d) an image processing mechanism for processing the sequentially acquired images of each of the plurality of the illuminated fields of view in each of a plurality of the wafer dies and detecting a wafer defect by comparing the sequentially acquired images using a die-to-die comparison method.
According to the present invention, there is provided an electro-optical camera for inspecting a patterned semiconductor wafer of dies for a defect, comprising a focal plane assembly including at least one detector ensemble, the detector ensemble includes an array of at least two two-dimensional matrix photo-detectors operating with a mechanism for simultaneous acquisition of images of each of a plurality of illuminated fields of view in each of a plurality of the wafer dies.
Implementation of the method and system of the present invention involves performing or completing tasks or steps manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of a given wafer inspection system, several steps of the present invention could be implemented by hardware or by software on any operating system of any firmware or a combination thereof. For example, as hardware, indicated steps of the invention could be implemented as a chip or a circuit. As software, indicated steps of the invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In any case, indicated steps of the method of the invention could be described as being performed by a data processor, such as a computing platform for executing a plurality of instructions.